The invention relates to a zoom lens barrel, and more particularly, to a zoom lens barrel having the capability to correct a diaphragm aperture in interlocked relationship with a change in the focal distance which occurs during a zooming operation.
Zoom lens barrels which exhibit an open F-value which varies during a zooming operation when a diaphragm mechanism is made to prevent a change from occurring in the diaphragm aperture during a zooming operation include (1) a barrel without a correction mechanism, (2) a barrel which maintains a constant F-value by changing the diaphragm aperture mechanically during a zooming operation, and (3) a barrel, as disclosed in Japanese Laid-Open Patent Application No. 24,134/1974 and Laid-Open Utility Model Application No. 8,226/1978, in which a diaphragm aperture is permitted to vary during a zooming operation when an open photometry is made, but in which when a release causes the aperture to be reduced, a given F-value is achieved by accomplishing a correction during the open photometry by means of a diaphragm value transmitting member.
In a zoom lens barrel of the type (1), a scale on a diaphragm ring is given in terms of F-value for a given focal length, and hence accurate F-values cannot be read for other focal distances. Specifically, if an exposure factor is determined by viewing a meter located within a finder, the factor must be adjusted after a zooming operation. When it is combined with a camera which has an AE (automatic exposure) lock, a zooming operation subsequent to the lock which is performed by the photometry causes a change in the F-value, resulting in an aberrant exposure value. In particular, this represents a major disadvantage where the result of a spotwise photometry is stored since it is a frequent occurrence that if an accurate photometry is made toward the telephoto end, a picture is actually taken by zooming toward the wide-photo end.
A zoom lens barrel of the type (2) reduces a region of bright F-values toward the wide-photo end, which can be effectively utilized in the optical design, to F-values which are in excess of open F-values toward the telephoto end. Thus a sacrifice is made of a bright region which facilitates a focussing operation, thereby presenting difficulties in focussing within a finder and a framing operation. This caused inconveniences in a photographing mode which makes a maximum advantage of a bright F-value region toward the wide-photo end or in a programmed photographing mode which does not require a correction of F-values.
Considering a zoom lens barrel of the type (3), it may be assumed that an AE lock operates in following relationship with a shutter dial and a diaphragm value transmitting member. By way of example, assume that a nominal F4 lens is used having an open F-value which is equal to F4 toward the telephoto end and to F2.8 toward the wide-photo end, and that a diaphragm aperture of F5.6 is established by means of a diaphragm ring. When the AE lock operates toward the telephoto end, the diaphragm value transmitting member will be brought to a position corresponding to F5.6. If a zooming operation now takes place to bring the camera toward a wide angle condition, the open F-value will be equal to F2.8, and the diaphragm value transmitting member will shift to a position which is equivalent to F8 toward the telephoto end. Since the diaphragm ring is located at the position of F5.6, a shutter release operation causes the diaphragm aperture to be reduced to a condition corresponding to F5.6 under the wide angle condition. However, because the camera operates on the basis of information available from the telephoto end, a shutter speed will be established in a manner corresponding to F8, disadvantageously resulting in an overexposure by 1 EV. In addition, this barrel suffers from the problem of limited F-value at the maximum aperture in a similar manner to a lens barrel of the type (2). Finally, in an arrangement in which an open F-value information is varied during a zooming operation before it is transmitted to a camera body, the mechanism or electrical circuit required become complex, resulting in an increased cost.
Thus, there has been a need in the art for a zoom lens barrel which has the combined functions of correcting the open F-value so as to be maintained constant during a zooming operation and of terminating the correction function to permit the maximum diaphragm aperture to be maintained. To this end, (4) an arrangement has been recently proposed (Japanese Laid-Open Patent Application No. 10,910/1984) which has a correction mechanism to maintain an open F-value constant and a mechanism to maintain a diaphragm aperture and in which both mechanisms can be selectively switched over the entire range of zooming operation. However, in this arrangement, the occurrence or not of the correction takes place in response to the intent of a photographer which is activated in terms of a devoted manual member. Thus, a photographer must operate this manual member in addition to the diaphragm ring, which is troublesome in operation, and it is expected that a beginner may fail to operate it properly, thus standing in the way of a satisfactory photographing operation.